I've seen estimates of around $150 per square foot, at least for this area. But it could go much higher, too.
Prices tend to run from a low of $125 to upwards $175 per square foot. But that number can be very deciving. Local costs can vary, so can disposal costs, supply costs, and prep costs. Does the electrical system support the addition? What about the heat and cooling system. Does the house support the 2nd floor? What about the footings?
My suggestion is to get drawings done professionally then use them to get your proper costs. Square foot costs are very deceptive and can end up costing you several thouands of dollars more once you start the project that you were not counting on. And with any price quote you do get, plan on 20% on overages that simply were not planned upon or surprises that always crop up.